Glaucoma is a medical condition in which the intraocular pressure of the eye increases. The anterior chamber, the cavity located between the cornea and lens, is filled with a fluid called aqueous humor. The aqueous humor drains from the anterior chamber to the venous system through the canals of Schlemm and is replaced continuously by secretions from the ciliary body. Glaucoma occurs when aqueous humor does not drain properly from the anterior chamber, causing an increase in intraocular pressure, closure of surrounding blood vessels, and damage to the retina and optic nerve. This condition, left untreated, can be very painful and lead to blindness. It is estimated that 65 million people world-wide suffer from this condition (Glaucoma Research Foundation, 2002). Animals can also be affected by this condition. It is estimated there are 65 million dogs in North America, of which approximately 1.3 million will develop glaucoma.
Closed-angle glaucoma occurs when the iris becomes misshapen and blocks the canals of Schlemm. The underlying cause of open-angle glaucoma is a blockage within the canals. About 3 million people in the United States are afflicted with open-angle glaucoma (Glaucoma Research Foundation, 2002). Secondary glaucoma, either open- or closed-angle, can be caused by injury, abnormal structures, inflammation, tumours, certain drugs, or diseases (Moffett et al, 1993).
Current treatments consist of medications, laser surgery and implantable drainage devices (Glaucoma Research Foundation, 2002). Medications, often in the form of eye drops, work to decrease intraocular fluid production, increase fluid drainage, or both. These medications are associated with side effects, including burning sensations in the eye, headaches, cardiac fluctuations and blurred vision. The administration of medications for the treatment of animals is difficult and ineffective. Laser surgery is used to open the fluid draining channels or correct structures in the eye. Multiple surgeries are often required, and medication is normally still used to help control intraocular pressure post-surgery. There is also the risk of allowing too much fluid to drain from the anterior chamber, resulting in hypotony and flattening of the eye.
Implants have also been used in the treatment of glaucoma. These implants drain aqueous humor from the anterior chamber into mesenchymal or subconjunctival tissues surrounding the eye. The aqueous humor of eyes afflicted with glaucoma has a relatively high concentration of cytokines. Cytokines are proteins that mediate the generation of an immune response. Implants that drain aqueous humor into tissues surrounding the eye induce scarring in such tissues, which scarring ultimately inhibits absorption of fluid from the shunt. Consequently glaucoma recrudesces and visual impairment redevelops.
Current drainage implants are classified as either restrictive or non-restrictive flow devices (Gal, 1999). Non-restrictive flow devices, such as the Molteno drainage implant, rely on the formation of fibrous tissue to slow the drainage of fluid from the eye. These implants require at least two invasive surgical procedures. The initial surgery is required to implant the device. Fibrous tissue generally takes several weeks to form, making it necessary to clamp or tie the device to limit fluid drainage and prevent hypotony. A second surgery is required to unclamp or untie the device once fibrosis has occurred. Restrictive flow devices, such as the Krupin Eye Disk, are designed with components that respond to fluid pressure, see for example U.S. Pat. No. 5,454,796, issued Oct. 3, 1995 to Krupin, and U.S. Design Pat. No. Des. 356,867 issued Mar. 28, 1995 to Krupin. When pressure in the drainage tube exceeds a certain limit, the component, or valve, will open and allow fluid to drain. Several problems have occurred with these devices. First, it is difficult to calibrate the valve to maintain an optimal intraocular pressure, and often leads to hypotony. Second, the formation of fibrous tissue can occlude the valve and render the drainage device useless. A similar restrictive flow device is disclosed in U.S. Pat. Nos. 6,142,969 and 6,007,510, issued to Nigam.
The frontal sinus 16 is an epithelium-lined cavity which is not affected by cytokines. Studies by Cullen et al., with a shunt implanted in dogs between the anterior chamber and the frontal sinus showed no evidence of bacterial movement from the frontal sinus 16 to the eye through this device (see Cullen et al, 1998).
Although somewhat successful in less severe cases, the prior art implant devices, while successful in the short term, are not effective for long-term management of glaucoma due to occurrence of hypotony or more difficult forms of glaucoma. More effective treatment regimes are needed.